Process for separating off nitrogen

ABSTRACT

The invention relates to a process for separating off C 2+ -hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, wherein
         a) the feed fraction ( 1, 20 ) is partially condensed (E 1,  E 1′,  E 3 ) and separated by rectification (T) into a C 2+ -hydrocarbon-rich fraction ( 11 ) and a C 2+ -hydrocarbon-depleted fraction ( 2 ),   b) the C 2+ -hydrocarbon-depleted fraction ( 2 ) is partially condensed (E 2 ) and separated into a liquid fraction which forms at least in part the reflux ( 3 ) for the separation by rectification (T), and a C 2+ -hydrocarbon-depleted gas fraction ( 4 ), and   c) the C 2+ -hydrocarbon-depleted gas fraction ( 4 ) is separated in a double-column process (N) into a nitrogen-rich fraction ( 8′ ) and a methane-rich fraction ( 7″ ).       

     According to the invention, the liquid fraction obtained in process step b) is fed ( 10 ) at least in part likewise to the double-column process (N) and separated therein into a nitrogen-rich fraction ( 8′ ) and a methane-rich fraction ( 7″ ).

SUMMARY OF THE INVENTION

The invention relates to a process for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, wherein

-   -   a) the feed fraction is partially condensed and separated by         rectification into C₂₊-hydrocarbon-rich fraction and a         C₂₊-hydrocarbon-depleted fraction,     -   b) the C₂₊-hydrocarbon-depleted fraction is partially condensed         and separated into a liquid fraction which forms at least in         part the reflux for the separation by rectification, and a         C₂₊-hydrocarbon-depleted gas fraction, and     -   c) the C₂₊-hydrocarbon-depleted gas fraction is separated in a         double-column process into a nitrogen-rich fraction and a         methane-rich fraction.

A process of the type in question for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons is known, for example, from U.S. Pat. No. 4,664,686. With reference to FIG. 1, which essentially corresponds to FIG. 3 of the abovementioned U.S. patent, a description will be given hereinafter of the process of the type in question for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons.

Via line 1, a feed fraction containing essentially nitrogen and hydrocarbons and which originates, for example, from an oil degassing or LNG plant which is not shown in the FIG. 1, is introduced into heat exchanger E1. The feed fraction (petroleum associated gas or light expansion gas) preferably has a pressure of above 25 bar. It has optionally already been subjected to a pretreatment, such as desulphurization and/or drying. In the heat exchanger E1, the feed fraction is cooled and partially condensed against process streams which will be considered in more detail hereinafter. Via line 1′, the partially condensed feed fraction is taken off from the heat exchanger E1 and, via the expansion valve a, introduced into a separation column T.

Separating off nitrogen from a feed fraction containing essentially nitrogen and hydrocarbons by means of a double-column process, as will be described hereinafter, customarily requires a nitrogen content in the feed fraction of at least 30% by volume. This minimum nitrogen content is necessary in order to be able to achieve the customarily required purities for the product streams nitrogen, the methane content of which should be less 0.1% by volume, and natural gas or methane, the nitrogen content of which should be less than 5% by volume, which product streams are obtained by the double-column process.

If the nitrogen content in the feed fraction falls below the abovementioned minimum nitrogen content at times or fundamentally, enrichment of the nitrogen concentration in the feed fraction before it is fed into the double-column process is necessary or desirable. The abovementioned separation column T serves for this purpose. By means of the separation column T, a low-nitrogen, C₂₊-rich hydrocarbon fraction is separated off from the feed fraction, which C₂₊-rich hydrocarbon fraction is taken off from the bottom of the separation column T via the line 5, cold-producingly expanded in the valve b and, after warming and vaporization in the heat exchanger E1, is released via line 5′ as what is termed a medium-pressure hydrocarbon fraction. A substream of this liquid fraction is taken off from the bottom of the separation column T, and, after a cold-producing expansion in the valve c, is added via the line 6 to the methane-rich fraction taken off from the double-column process N, and thus serves for providing cold in the top condenser E2. This methane-rich fraction will be considered in more detail hereinafter.

Via line 2, from the top of the separation column T, a C₂₊-hydrocarbon-depleted fraction is taken off, which fraction has a higher nitrogen content compared with the feed fraction introduced in the line 1. This C₂₊-hydrocarbon-depleted fraction is partially condensed in the heat exchanger or top condenser E2 and fed to the separator D via the line 2′. From the bottom of the separator D, via line 3, the liquid fraction occurring is taken off and fed as reflux to the column T. Generally, a return pump P must be provided in the line 3. This can be omitted if the separator D is arranged above the feed-in point of the reflux stream.

The C₂₊-hydrocarbon-depleted gas fraction removed from the separator D is fed via line 4 to a double-column process N which is shown only schematically. Such double-column processes are sufficiently known to those skilled in the art from the prior art. A prior art double-column process is described, for example, in the German patent application 10 2009 008229 which was not published before the priority date of the present application. By citing the German patent application 10 2009 008229, the contents thereof are hereby fully incorporated into the contents of the present patent application.

The bottom of the separation column T is heated by means of a bottom heater integrated into the heat exchanger E1—shown by the pipe sections 9 and 9′.

The nitrogen-rich fraction obtained in the double-column process N is removed via line 8, warmed in the heat exchanger E1 against the feed fraction to be cooled, and then fed via line 8′ to a further use thereof. The methane-rich fraction obtained in the double-column process N is fed via line 7 to the top condenser E2—if appropriate after previous addition of a substream of the liquid fraction taken off in the separation column T—warmed therein and vaporized, at least in part, subsequently fed via line 7′ to the heat exchanger E1, and, after further warming and complete vaporization against the feed fraction to be cooled, is fed via line 7″ to further use thereof.

In a process as described with reference to FIG. 1, the main feature is directed towards optimizing the flow rate of the C₂₊-hydrocarbon fraction removed from the bottom of the separation column T in order to be able to release this at elevated pressure via the lines 5 and 5′. The remaining hydrocarbons are released at a lower pressure via the line 7″ in this procedure. Since both hydrocarbon fractions 5′/7″ are to be released together, it is necessary to compress at least one of the two fractions, customarily fraction 7″, to the desired release pressure—this compression is not shown in FIG. 1. For this reason, the composition of the liquid fraction 5 taken off from the bottom of the separation column T is optimized to a low nitrogen content. The composition of the gas fraction taken off from the top of the separation column T, in contrast, is optimized to a nitrogen content as high as possible, but not in respect of the hydrocarbon composition, in particular a high methane content.

If compression of the hydrocarbon fraction(s) is to be avoided, release of the hydrocarbon fractions at a pressure which is uniform and simultaneously as high as possible must be sought. The methane-rich stream taken off from the double-column process N in this case must be set such that it can fulfil its tasks in the heat integration at a pressure as high as possible.

Thus, an aspect of the present invention is to provide a process of the type mentioned above, for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, which avoids the disadvantages described.

This aspect can be achieved according to the invention by a process for separating off C₂₊-hydrcarbons from a feed fraction containing essentially nitrogen and hydrocarbons, wherein the liquid fraction, obtained from the partial condensation of the C₂₊-hydrocarbon-depleted fraction removed from the top of the separation column T, is fed at least in part, together with the C₂₊-hydrocarbon-depleted gas fraction, to the double-column process, and is separated therein into a nitrogen-rich fraction and a methane-rich fraction.

Upon further study of the specification and appended claims, further aspects and advantages of this invention will become apparent to those skilled in the art.

Further advantageous configurations of the process according to the invention for separating off C₂₊-hydrcarbons from a feed fraction containing essentially nitrogen and hydrocarbons, which are subjects of dependent claims, are characterized in that

-   -   the feed fraction containing essentially nitrogen and         hydrocarbons is divided into a plurality of substreams, these         substreams are partially condensed separately from one another,         and then separated by rectification,     -   the cooling of the substreams of the feed fraction proceeds in         double-pipe heat exchangers, preferably in helically coiled heat         exchangers, wherein the cooling or partial condensation of the         substreams proceeds preferably in the tubes and the vaporization         or warming of the cold fractionation products proceeds on the         shell side of the helically coiled heat exchangers and/or         preferably the cooling or partial condensation of the substreams         proceeds in an ascending manner on the tube side and the warming         or vaporization of the fractionation products proceeds in a         falling manner on the shell side, and     -   at least one of the substreams of the feed fraction is separated         into a gas fraction and a liquid fraction and these are fed         separately from one another to the separation by rectification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and further details, such as features and attendant advantages, of the invention are explained in more detail below on the basis of the exemplary embodiments which are diagrammatically depicted in the drawings, and wherein:

FIG. 1 schematically illustrates an exemplary embodiment of the prior art; and

FIGS. 2 and 3 each illustrate exemplary embodiments of the process according to the invention.

The process according to the invention for separating off C₂₊-hydrcarbons from a feed fraction containing essentially nitrogen and hydrocarbons and also other configurations thereof will be described in more detail hereinafter with reference to the exemplary embodiments shown in FIGS. 2 and 3. Hereinafter, in the explanation of the exemplary embodiments shown in FIGS. 2 and 3, only the differences from the procedure as shown in FIG. 1 will be considered.

As shown in FIGS. 2 and 3, the liquid separated in the separator D is fed according to the invention, in part, via line 10 and expansion valve d to the double-column process N. The remaining part of the liquid from separator D is fed to the separation column T via line 3 as reflux stream.

Due to the introduction, according to the invention, of the above-described liquid fraction into the double-column process N, the energy balance thereof is altered in such a manner that the methane-rich stream taken off from the double-column process N via line 7 is completely liquid, instead of partially vaporized as in previous systems. As a result, sufficient cold capacity is available to the top condenser E2 even without the addition, shown in FIG. 1, of a substream of the liquid fraction taken off from the bottom of the separation column T. The liquid fraction taken off from the bottom of the separation column T via line 11 is therefore, after expansion in the valve e, added to the methane-rich fraction between the top condenser E2 and heat exchanger E1. The liquid fraction taken off via line 11 is therefore advantageously only used for precooling the feed fraction in the heat exchanger E1. In the procedure according to the invention, the required temperature profile of the overall process can therefore be provided by hydrocarbons vaporizing essentially isobarically—this means that only conventional pressure drops occur of in total a maximum of 1 bar in the heat exchangers E2 and E1.

By means of the procedure according to the invention, the fraction taken off via line 2 from the top of the separation column T is now freed as far as possible C₂₊-hydrocarbons and carbon dioxide. The methane-rich stream taken off from the double-column process N via line 7 thus has a significantly higher methane content than that in the procedure shown in FIG. 1. Advantageously, the operation of the separation column T is optimized to the effect that the content of C₂₊-hydrocarbons in the fraction taken off via line 2 from the top of the separation column T is a maximum of 0.1% by volume (1000 vppm), preferably a maximum of 0.01% by volume (100 vppm).

The process procedure shown in FIG. 3 differs from that shown in FIG. 2 essentially in that the multipipe heat exchanger E1 is divided into a plurality of double-pipe heat exchangers E1, E1′ and E3. In addition, an additional separator D′ is provided. Such a process procedure makes it possible to ensure stable flow conditions in the heat exchangers in a wide range of feed fraction composition and load states.

In the embodiment of FIG. 3, the feed fraction is divided into two substreams 1 and 20. Both are cooled and partially condensed in the heat exchangers E1 and E1′, respectively. The first substream 1 is fed in a known manner to the separation column T via line 1′ and expansion valve a. The second substream 20 is fed to the heat exchanger E3 via line 20′ and subsequently separated in the separator D′ into a liquid fraction and a gas fraction.

For the purpose of heating the bottom of the separation column T, a hydrocarbon-rich fraction is removed from separation column T via line 30 at a suitable point, warmed in a heat exchanger E3 and also partially vaporized and then fed via line 30′ to the separation column T.

The gaseous fraction taken off via line 22 from the separator D′ is cooled in the heat exchanger E1′, partially condensed and subsequently fed via line 22′ and expansion valve f to the separation column T. Via the choice of the position of the feed-in points of the fractions in the lines 1′, 21 and/or 22′, the operation of the separation column T can be varied or optimized.

The above-described heat exchangers E1 and E1′ are advantageously constructed as helically coiled heat exchangers, wherein the cooling or partial condensation of the feed fractions proceeds in the tubes and the vaporization or warming of the cold fractionation products proceeds on the shell side of the helically coiled heat exchangers. In addition, the cooling or partial condensation of the feed fraction preferably proceeds in an ascending manner on the tube side and the warming or vaporization of the fractionation products proceeds in a falling manner on the shell side.

If the top condenser E2 is constructed as a circulation evaporator, complete vaporization of the methane-rich fraction removed from the double-column process N via line 7 can be achieved in a controlled manner. From this circulation vessel having a controlled liquid level in which the top condenser E2 is arranged, the fraction 7′ is thereby taken off exclusively in the gaseous state.

The process according to the invention for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons makes it possible to achieve a procedure in which now only one hydrocarbon-rich fraction can be obtained at a comparatively high pressure level and utilized for providing cold, and so generally recompression of this fraction is unnecessary.

The entire disclosure[s] of all applications, patents and publications, cited herein and of corresponding German Application No. DE 10 2009 036366.1, filed Aug. 6, 2009, are incorporated by reference herein.

The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions. 

1. A process for separating off C₂₊-hydrocarbons from a feed fraction containing essentially nitrogen and hydrocarbons, said process comprising: a) partially condensing (E1, E1′, E3) said feed fraction (1, 20) and separating said feed fraction by rectification in rectification column (T) into a C₂₊-hydrocarbon-rich fraction (11) and a C₂₊-hydrocarbon-depleted fraction (2), b) partially condensing (E2) said C₂₊-hydrocarbon-depleted fraction (2) and separating said C₂₊-hydrocarbon-depleted fraction into a liquid fraction which forms at least in part reflux (3) for the separation by rectification (T), and a C₂₊-hydrocarbon-depleted gas fraction (4), c) separating said C₂₊-hydrocarbon-depleted gas fraction (4) in a double-column separator (N) into a nitrogen-rich fraction (8) and a methane-rich fraction (7), and wherein at least part of said liquid fraction obtained in process step b) is fed (10) to said double-column process (N) and is separated in said double-column separator (N) into said nitrogen-rich fraction (8) and said methane-rich fraction (7).
 2. The process according to claim 1, wherein said feed fraction is divided into a plurality of substreams (1, 20), and said plurality of substreams are partially condensed (E1, E1′, E3), separately from one another, and then separated by rectification in said rectification column (T).
 3. The process according to claim 2, wherein cooling of the substreams (1, 20) of said feed fraction is performed in double-pipe heat exchangers (E1, E1′, E3).
 4. The process according to claim 3, wherein the cooling or partial condensation of the substreams (1, 20) proceeds in the tubes of said double-pipe heat exchangers and the vaporization or warming of cold fractionation products (7′, 8) proceeds on the shell side of said double-pipe heat exchangers.
 5. The process according to claim 3, wherein the cooling or partial condensation of the substreams (1, 20) proceeds in an ascending manner on the tube side of said double-pipe heat and the warming or vaporization of fractionation products (7′, 8) proceeds in a falling manner on the shell side of said double-pipe heat.
 6. The process according to claim 4, wherein the cooling or partial condensation of the substreams (1, 20) proceeds in an ascending manner on the tube side of said double-pipe heat and the warming or vaporization of fractionation products (7′, 8) proceeds in a falling manner on the shell side of said double-pipe heat.
 7. The process according to claim 3, wherein said double-pipe heat exchangers are helically coiled heat exchangers (E1, E1′).
 8. The process according to claim 4, wherein said double-pipe heat exchangers are helically coiled heat exchangers (E1, E1′).
 9. The process according to claim 5, wherein said double-pipe heat exchangers are helically coiled heat exchangers (E1, E1′).
 10. The process according to claim 6, wherein said double-pipe heat exchangers are helically coiled heat exchangers (E1, E1′).
 11. The process according to claim 2, wherein at least one of the substreams (1, 20) of said feed fraction is separated (D′) into a gas fraction (22) and a liquid fraction (21), and said gas fraction (22) and liquid fraction (21) are fed separately from one another to the separation by rectification in said rectification column (T).
 12. The process according to claim 1, wherein liquid from the bottom of said rectification column T is not introduced into said double-column separator N.
 13. The process according to claim 1, wherein said methane-rich fraction (7, 7′) removed from said double-column separator (N) is heated in a top condenser of said rectification column T and further heated in a heat exchanger against said feed fraction (1, 20).
 14. The process according to claim 13, wherein a liquid fraction (11) is removed from the bottom of said rectification column T, expanded, and added to said methane-rich fraction at a point between said top condenser (E2) and said heat exchanger (E1).
 15. The process according to claim 1, wherein the content of C₂₊-hydrocarbons in said C₂₊-hydrocarbon-depleted fraction (2) is at most 0.1% by volume (1000 vppm).
 16. The process according to claim 15, wherein the content of C₂₊-hydrocarbons in said C₂₊-hydrocarbon-depleted fraction (2) is at most 0.01% by volume (100 vppm). 